LTP's Optical Metrology System

The LTP phasemeter (designed and built at Birmingham University) is a custom built measurement device which takes photocurrents from the 4 quadrants of each of the 8 quadrant photodiodes, digitises the signals at 100Hz using 16-bit Analog-to-digital converters, and from these 32 signals, performs a single-bin discrete Fourier transform (SBDFT) at the prescribed heterodyne frequency. The components of the Fourier transfer computations (dc term, sine term, and cosine term) are transported along the internal Mil-bus to the DMU, the signal processing computer of the LTP.